This invention deals with devices for converting rotary motion into recilinear motion, and is directed more specifically to such a rotary-to-linear converter of the type having a worm and a ball nut, with a plurality or multiplicity of antifriction balls rollably engaged between their opposed screw threads to translate the rotation of either into the rectilinear travel of the other. The invention also specifically concerns feed mechanisms incorporating one or two such rotary-to-linear converters.
Rotary-to-linear converters of the worm-and-ball-nut type, also described as the recirculating ball type in the art, convert sliding friction into rolling friction by virtue of the rolling balls. Because of the minimal frictional resistance between the mating parts, the devices of this class have found extensive use as feed mechanisms of machine tools and other pieces of machinery.
As heretofore constructed, however, the worm-and-ball-nut assemblies have possessed some drawbacks. One of these concerns the tubular ball return guides conventionally employed for recirculating the balls. After making from one and a half to three revolutions around the worm, the balls enter the return guide and roll back to the initial point between the worm and the ball nut by making approximately one turn around the ball nut. The ball return guides are curved with a radius less than that of the ball nut. Such curves adversely affect the smooth travel of the balls through the return guides and give rise to noise. The smooth recirculation of the balls is a prerequisite for the efficient, frictionless reative rotation and axial movement of the worm and the ball nut.
Another problem with the prior art is the axial play or backlash between the worm and the ball nut. The backlash is an unfailing cause of uneven wear, noise, and the poor response with which either of the worm and the ball nut starts travelling axially upon rotation of the other. A conventional remedy for this defect has been to split the ball nut into a pair of halves, along a plane normal to its axis, and to place an intermediate ring therebetween for preloading the two groups of balls on its opposite sides either toward or away from each other. The splitting of the ball nut necessitates, of course, the reinforcement of the ball nut assembly including the intermediate ring, thus making difficult the manufacture and assemblage of the device.
A further disadvantage of the known rotary-to-linear converters has manifested itself in rotatably mounting the ball nut to some external part. The antifriction bearing used for this purpose is almost invariably of the rolling-element type, comprising one or more rows of rolling elements, normally balls, interposed between two concentric rings or races. Placed around the ball nut, the bearing of this conventional design makes the complete device inconveniently bulky for use as a feed mechanism.
It is also a disadvantage of the prior art that the opposite axial ends of the ball nut have been left open, inviting the intrusion of dust and other foreign matter into the tubular space between the worm and the ball nut. The accumulation of dust or the like on the opposed surfaces of the worm and the ball nut seriously impedes the smooth rolling of the balls along their threads and, in consequence, the smooth relative motion of the worm and the ball nut.
An additional problem appears in the use of the worm-and-ball-nut assembly as a feed mechanism. According to a typical conventional feed mechanism of this type the ball nut is coupled to an object to be fed, such as the table of a machine tool. The worm, on the other hand, is geared at one end to a drive motor and is journaled at the other end in a bearing, with the drive motor and the bearing being both supported on a stationary part. Thus the rotation of the worm results in the axial movement of the ball nut together with the object coupled thereto. This prior art construction invites the problem of the critical speed of the worm, that is, the angular speed at which the worm becomes dynamically unstable with large lateral amplitudes, due to resonance with the natural frequencies of lateral vibration of the worm. The worm must therefore be revolved at safely less than the critical speed even for moving the desired object at high speed, thus setting a limit on the quick feeding of the object.
It has been suggested to circumvent worm resonance by making the lead angles of the screw threads of the worm and the ball nut very large, to such an extent that the desired object may be fed at high speed at a relatively low revolving speed of the worm. Screw threads with such a great lead angle are, however, very difficult to cut on the inner face of the ball nut with any acceptable degree of accuracy. Further an increase in the lead angle leads to the reduction of the force with which the ball nut is thrusted axially on the worm, and also makes difficult the accurate positioning of the object.